Avagadro's number

Question

Ive been reading about moles and I can't seem to understand avagadro's number. I get that the idea is 1g of hydrogen, 12g of carbon, and 16g of oxygen would all have the same number of atoms, but why? Is it just a coincidence, what's the reason behind this?

Answer 1

A mole is a measurement of the number of atoms in a substance. Specifically, it is equal to the number of atoms in the substance divided by some constant $x$. We define the molecular weight of a compound to be the number of grams of that substance that are required to make up a mole of that substance. Now, all that we need is to find some value for $x$ that makes it easy to discover the molecular weight of an atom/compound. It turns out that protons and neutrons have around the same mass, and electrons have negligible mass compared to the two of them. Because of this, we can define one unit of "mass" to be the mass of a neutron or proton, allowing us to find the molecular weight of some compound just by summing up all of the protons and neutrons in that compound. Since this is our molecular weight, it will be measured in units of $\dfrac{g}{mol}$. Now, we just need to find our $x$ such that our conception of a mole corresponds to a correct number of atoms. It turns out by experimental results that this number must be $6.022 * 10^{23} \dfrac{atoms}{mol}$, or avogardo's number.